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Thursday, April 5, 2012
Synthesis of Ammonia is one of the remarkable achievements of Chemical engineering in forties. It is a precursor for Urea, the fertilizer that brought about ‘Green revolution’ in agriculture industry and helped to achieve record food production all over the world. It was a milestone in modern chemistry to synthesis a molecule containing 1 atom of Nitrogen and 3 atoms of Hydrogen, reperesented by NH3 called Ammonia. The Heber-Bosch process for the production of Ammonia is a well established, mature, commercial technology. The process uses a Hydrocarbon source such as Naphtha or Natural gas as the feed stock to generate a synthesis gas composed of Hydrogen and Carbondioxide.The gas mixture is separated into carbon dioxide and Hydrogen using PSA (pressure swing adsorption ) technology. The resulting Hydrogen is used to combine with Nitrogen to synthesize Ammonia. The chemical reaction can be represented by the following equation. N2 + 3H2 ---------- 2 NH3 The above reaction takes place at a pressure of 100-200 bars and temperature of 300-500C in presence of a catalysts. It is an exothermic (heat releasing) reaction and the catalyst bed is cooled and maintained at 400C to be efficient.Buth this process of Hydrogen generation using Hydrocarbon emits greenhouse gases. Alternatively, Hydrogen can be generated using different methods using renewable energy sources using water electrolysis. Such process may be used in the future for this application. Nitrogen is derived from atmospheric air. The air we breathe contains about 79% of Nitrogen and 21% Oxygen. But these two gases can be separated by liquefying the air by cryogenic process and distilling them into two fractions. Alternatively, they can be separated using pressure swing adsorption or membrane separation process, utilizing their density differences. In either way, Nitrogen can be separated from atmospheric air. By combining the above Hydrogen and Nitrogen, it is possible to synthesis Ammonia on a commercial scale. The ammonia can be easily split into Hydrogen and Nitrogen by passing Ammonia through a bed of Nickel catalyst at 200-400C as and when required, to generate onsite Hydrogen. This Hydrogen can be used for power generation or to run our cars using PEM Fuelcell.As we have seen previosulsy, we are now looking for various sources of Hydrogen, and Ammonia is one of the promising sources for couple of reasons. The process and technology of Ammonia production, transportation and usage is well documented and has been practiced for few decades. It does not emit any greenhouse gases.Liquified Ammonia has been widely used in air-conditioning and refrigeration systems. Ammonia can be easily metered into any system directly from the cylinder. It is easier to use Ammonia directly into a convention internal combustion engines in place of Gasoline and this technology has already been practiced in 1880. Ammonia is pungent and any leakage can be easily identified. The advantage of using Ammonia as a fuel in cars, it does not emit any smoke but only water vapour.It can be admixed with Gasoline or used as 100% anhydrous Ammonia. It also helps in reduction of NO2 emission, especially is diesel engines. Ammonia has a great potential as a source of future fuels provided the sources of Hydrogen comes from water using renewable technologies or by photo-electrolysis using sunlight.